Webcast works in real-time to convert your live webinar into a single video stream, and then broadcasts it to attendees.
The hosting experience is exactly the same, but for attendees, the webinar is packaged into a single video that plays after a slight delay (approximately 5-10 seconds).
This approach reaches a broader range of attendees more reliably.
If you have attendees with "last-mile" issues like firewalls, VPNs, Wi-Fi settings, and browser plugins that inhibit their ability to receive audio and video streams in webinars, Webcast will solve most issues. It will also help those on slower internet connections or attending on older/out-of-date browsers like Internet Explorer.
Try it for yourself by Creating a Live Webinar, and enabling Webcast Mode in Advanced Settings (on your webinar's Edit page).
Currently it's a required setting for webinars with over 50 registrants: